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Applied and Environmental Microbiology, August 2003, p. 4971-4974, Vol. 69, No. 8
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.8.4971-4974.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
Identification and Application of Plasmids Suitable for Transfer of Foreign DNA to Members of the Genus Gordonia
Matthias Arenskötter, Dirk Baumeister, Rainer Kalscheuer, and Alexander Steinbüchel*
Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, D-48149 Münster, Germany
Received 29 October 2002/
Accepted 7 May 2003

ABSTRACT
Gene transfer systems for
Gordonia polyisoprenivorans strains
VH2 and Y2K based on electroporation and conjugation, respectively,
were established. Several parameters were optimized, resulting
in transformation efficiencies of >4
x 10
5 CFU/µg of
plasmid DNA. In contrast to most previously described electroporation
protocols, the highest efficiencies were obtained by applying
a heat shock after the intrinsic electroporation. Under these
conditions, transfer and autonomous replication of plasmid pNC9503
was also demonstrated to proceed in
G. alkanivorans DSM44187,
G. nitida DSM44499
T,
G. rubropertincta DSM43197
T,
G. rubropertincta DSM46038, and
G. terrae DSM43249
T. Conjugational plasmid DNA
transfer to
G. polyisoprenivorans resulted in transfer frequencies
of up to 5
x 10
-6 of the recipient cells. Recombinant strains
capable of polyhydroxyalkanoate synthesis from alkanes were
constructed.

INTRODUCTION
Since reclassification of the gram-positives
Rhodococcus aichiensis and
Nocardia amarae to the genus
Gordonia (
13), this taxon is
now a well-defined genus among the
Corynebacterium, Mycobacterium,
and
Nocardia (CMN) group of actinomycetes. Species of
Gordonia have attracted much interest in recent years due to their unusual
and diverse capabilities to catalyze biotransformations and
biodegradation of poorly approachable substances (
2,
7,
9,
16).
Although the number of reports of newly identified species of
this genus steadily increases, no suitable genetic transfer
systems have yet been described. Molecular analysis of rubber
degradation by
G. polyisoprenivorans and of other interesting
pathways of
Gordonia species is hampered by the lack of suitable
and efficient gene transfer systems. Therefore, the present
study identified plasmids, which can be transferred to
G. polyisoprenivorans and other species of this genus by conjugational transfer or
electroporation and which are stably maintained.

Identification of vector systems for G. polyisoprenivorans.
The 6.3-kbp plasmid pNC9503 (Fig.
1b) was recently described
as an
E. coli/
Rhodococcus shuttle vector (
12). It possesses
a unique restriction site for
XbaI and comprises the kanamycin
resistance gene from Tn
903 (
24) for selection in
E. coli and
Rhodococcus/
Gordonia. In addition, pNC9503 carries a thiostrepton
resistance gene from
Streptomyces azureus for selection in coryneform
bacteria. The origin of replication (
oriV) in actinomycetes
is located on a fragment derived from the native
Rhodococcus rhodochrous plasmid pNC903. A partial sequence revealed that
it was 90% similar to the sequence of the
R. rhodochrous plasmid
pRC4 (
10), which encodes a RepA and RepB protein. This plasmid,
in turn, shares sequence similarity with the
Mycobacterium fortuitum plasmid pAL5000 (
17). Plasmid pNC9501 (Fig.
1a) is a derivative
of pNC9503 differing from the latter only in possessing two
additional unique restriction sites for
KpnI and
EcoRI.
These vectors were introduced into
G. polyisoprenivorans strains
VH2 and Y2K applying a basic electroporation protocol previously
developed for
R. opacus PD630 (
12). Electroporation of strain
Kd2 failed. The electroporated cells were plated on media containing
25 µg of thiostrepton or 25 µg of kanamycin/ml for
the selection of transformants. Resistant colonies appeared
after 4 to 6 days of incubation at 30°C. Plasmid DNA was
isolated from each 20 randomly chosen transformants and then
analyzed with respect to their restriction patterns. All transformants
harbored plasmid DNA, indicating that autonomous replication
of both plasmids occurs in
G. polyisoprenivorans. They were
therefore suitable as
E. coli-
G. polyisoprenivorans shuttle
vectors. However, restriction analysis revealed that ca. 50%
of the plasmids recovered from the recombinant clones had undergone
identical modifications resulting in truncations of the 5.1-kbp
EcoRI fragment of plasmid pNC9503 by deletion of ca. 800 bp.
By changing the electroporation protocol, these modifications
were prevented (see below).
Because first electroporation experiments led to transformation rates of only about 103 transformants/µg of plasmid DNA, the electroporation protocols for both strains of G. polyisoprenivorans were systematically optimized. For this, one parameter of cultivation or of the electroporation conditions was altered at a time, whereas the others were kept constant. The optimum of the field strength was 10 kV/cm. Transformation efficiencies depended strongly on the cultivation conditions, the medium, and the type and concentration of cell wall-weakening additives. The most suitable basic medium to obtain electrocompetent cells was Luria-Bertani (LB) broth (18); LB broth was twofold more efficient than nutrient broth (ADSA-Micro, Barcelona, Spain) or standard I complex nutrient broth (Merck, Darmstadt, Germany). Highest transformation efficiencies were obtained if cells were used from the early growth phase when the cultures had reached optical densities of 0.5 at 600 nm. Therefore, all subsequent alterations of medium composition and cultivation conditions were done with LB medium, and the effects of sucrose, glycine, and isonicotinic acid hydrazide on transformation efficiency were investigated in a range previously described for Rhodococcus spp. (12). Glycine and sucrose in the medium enhanced the electroporation efficiency most effectively at concentrations of 0.5% (wt/vol) and 1.5% (wt/vol), respectively. Optimal concentration of isonicotinic acid hydrazide was 1.5 µg/ml; its addition increased the efficiency of electroporation about twofold. Plasmid DNA concentrations of
0.25 µg/ml resulted in the highest transformation rates. Temperatures and the duration of temperature shifts used for preincubation or incubation after the electroporation pulse also affected transformations. For G. polyisoprenivorans highest transformation efficiencies of up to 4 x 105 CFU/µg of plasmid DNA were obtained with cells grown at 30°C, and if they were incubated for 10 min at 0°C before and for 6 min at 46°C after the electroporation pulse. This heat shock also suppressed the 800-bp deletion of transformed plasmid DNA. The optimized electroporation protocol is as follows: DNA was purified from E. coli strains and dialyzed against distilled H2O by using microfilters (pore size of 0.025 µm; Millipore, Eschborn, Germany). For growth of G. polyisoprenivorans 50 ml of LB medium supplemented with 0.5% (wt/vol) glycine, 1.5% (wt/vol) sucrose, and 1.5 µg of isonicotinic acid hydrazide/ml in a 250-ml Erlenmeyer flask were inoculated with 1 ml of an overnight preculture in standard I complex nutrient broth medium, and the cells were grown at 30°C to an optical density of 0.5 at 600 nm. Cells were harvested, washed twice, and concentrated 20-fold in cold double-distilled H2O. Competent cells were either used directly for electroporation or stored at -70°C. Immediately before electroporation, 400 µl of competent cells were mixed with 0.001 to 10 µg of DNA and preincubated 10 min on ice. Electroporation with a model 2510 electroporator was performed in electrocuvettes (Eppendorf-Netheler-Hinz, Hamburg, Germany) with gaps of 2 mm and at the following settings: 10 kV/cm, 600
, and 25 µF. Time constants of 4 to 5 ms were reached. Pulsed cells were immediately diluted with 600 µl of LB, incubated for 6 min at 46°C, regenerated at 30°C for 4 h, and plated on appropriate selective media, and transformants were identified after 4 to 6 days of incubation. In controls, no spontaneous kanamycin-resistant colonies occurred. The survival rate without heat shock was 68% (VH2) and 63% (Y2K) after electroporation and dropped to 44% (VH2) and 36% (Y2K) if heat shock was applied. This protocol was also applied to 16 different strains belonging to 12 different species of the genus Gordonia (Table 1). The transformation efficiencies for the other Gordonia strains were significantly lower than for G. polyisoprenivorans VH2 and Y2K and ranged between 102 and 104 CFU/µg of plasmid DNA. Autonomous replication of plasmid pNC9503 was shown to occur in G. alkanivorans DSM44187, G. nitida DSM44499T, G. rubropertincta DSM43197T, G. rubropertincta DSM46038, and G. terrae DSM43249T.

Construction of mobilizable vectors for conjugational transfer.
Because efficiencies of plasmid DNA transfer by electroporation
decrease with increasing plasmid sizes (
23), transfer of vectors
by conjugation using
E. coli S17-1 as a donor for
G. polyisoprenivorans was also investigated. Two mobilizable vectors were constructed.
(i)
oriV, comprising a 2.4-kbp
EcoRI/
HindIII fragment of plasmid
pNC9503, which mediates stable replication in
G. polyisoprenivorans,
was cloned into
EcoRI/
HindIII-digested DNA of the gram-negative
broad-host-range vector pBBR1MCS-2 (
14) (GenBank accession no.
U23751), yielding plasmid pBBRKmNC903 (Fig.
1c). (ii) A 1.7-kbp
BglII fragment containing the
cos sites enabling lambda packaging
of large DNA molecules for creating genomic libraries was derived
from vector pHC79 (
11) (GenBank accession no.
L08873). It was
treated with mung bean nuclease and subsequently cloned into
SmaI-digested pBBR1MCS-2 DNA, yielding pBBR1MCS-2cos (data not
shown). Afterward, the 2.4-kbp
EcoRI/
HindIII restriction fragment
of plasmid pNC9503 containing the
oriV of pNC903 was cloned
into
EcoRI/
HindIII-digested pBBR1MCS-2cos DNA, yielding pOpaCOS
(Fig.
1d). Applying a protocol described previously (
6), recipient
transfer frequencies of 6
x 10
-7 for vector pBBRKmNC903 and
5
x 10
-6 for vector pOpaCOS were obtained.

Recombinant biosynthesis of polyhydroxyalkanoates (PHAs) in G. polyisoprenivorans.
To analyze the suitability of these plasmids for transfer and
heterologous expression of foreign genes in
G. polyisoprenivorans,
recombinant strains of
G. polyisoprenivorans VH2 and Y2K capable
of PHA synthesis were constructed. Substrates of PHA
MCL synthase
(3-hydroxyacyl-coenzyme A) are available from ß-oxidation
when the cells grow on
n-alkanes. Furthermore, PHA biosynthesis
was previously reported for various species of the closely related
genus
Rhodococcus (
1,
8) and could be established in recombinant
strains of
R. opacus. When pAK71 (
12) harboring
phaC1 from
Pseudomonas aeruginosa was introduced into VH2 and Y2K, the recombinant
strains accumulated PHAs, contributing up to 8.3 or 13.2%, respectively,
of the cell dry matter during cultivation on mineral salts medium
under conditions of N starvation on long-chain
n-alkanes (
20).
Gas chromatography (GC) and GC-mass spectrometry (MS) analysis
of accumulated PHAs (
3) revealed that copolyesters mainly consisting
of odd-numbered 3-hydroxyalkanoates (3HHp, 3HHN, 3HUD, and 3HTD;
>88 mol%) were synthesized from pentadecane, whereas PHAs
mainly consisting of even-numbered 3-hydroxyalkanoates (3HO,
3HD, and 3HDD;

75 mol%) were synthesized from hexadecane (Table
2).
View this table:
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TABLE 2. PHA accumulation by recombinant strains of G. polyisoprenivorans VH2 and Y2K after cultivation in media containing different carbon sourcesa
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Conclusions.
The present study succeeded in establishing and optimizing two
different gene transfer systems for the rubber-degrading, gram-positive
bacterium
G. polyisoprenivorans strains VH2 and Y2K and several
other members of the genus
Gordonia based on electroporation.
Furthermore, conjugational plasmid transfer with
E. coli S17-1
as the donor, enabling the transfer of large constructs as required
for the phenotypic complementation of mutants, was established.
This is the first description of genetic transfer of DNA and
maintenance of foreign plasmids for various species of the genus
Gordonia. It will make these bacteria accessible for genetic
engineering, complementation of mutants, and heterologous expression
of genes to reveal the molecular and biochemical basis of interesting
metabolic pathways of
Gordonia species. Transformation efficiencies
of up to 4
x 10
5 CFU/µg of plasmid DNA are sufficiently
high to comply with the demands of standard genetic techniques
and resemble those reported for
R. opacus (
12),
Rhodococcus sp. strain TE1 (
21),
R. fascians (
5), and
Clavibacter michiganensis subsp.
sepedonicus (
15). The application of a heat shock after
electroporation increased transformation efficiencies, as reported
for
Corynebacterium glutamicum (
25), and prevented the specific
deletion of introduced plasmid DNA. Presumably, both effects
were due to the inactivation of a restriction system (
19,
25).
The newly established electrotransformation protocol was successfully
applied to establish a functional active PHA synthase of
P. aeruginosa in
G. polyisoprenivorans, resulting in PHA
MCL biosynthesis
from
n-alkanes. The
E. coli lacZ promoter of pAK71 located upstream
of
phaC1 was obviously recognized by the
G. polyisoprenivorans RNA polymerase. Since PHA
MCL biosynthesis did not depend on
IPTG (isopropyl-ß-
D-thiogalactopyranoside) addition,
G. polyisoprenivorans obviously does not produce a
lac repressor,
and
lacZ promoter dependent genes are constitutively expressed.

FOOTNOTES
* Corresponding author. Mailing address: Institut für Molekulare Mikrobiologie und Biotechnologie, Westfälische Wilhelms-Universität Münster, Corrensstrasse 3, D-48149 Münster, Germany. Phone: 49 (251) 8339821. Fax: 49 (251) 8338388. E-mail:
steinbu{at}uni-muenster.de.


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Applied and Environmental Microbiology, August 2003, p. 4971-4974, Vol. 69, No. 8
0099-2240/03/$08.00+0 DOI: 10.1128/AEM.69.8.4971-4974.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.
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